Antioxidants are substances that can reduce negative changes in fat. Many antioxidants are very effective during storage, but during heating they lose their properties. It is very important to find antioxidants that will be stable at high temperatures and protect fat throughout the entire frying process. The aim of this study was to estimate the effect of natural and synthetic antioxidants on changes in phytosterols of rapeseed oil during heating. Oil with antioxidants was heated at 180 °C for 4 h in a Rancimat® and in an Oxidograph®. Ethanol extract of rosemary, ethanol extract of green tea, and BHT were used in the study. The contents of phytosterols (sitosterol, campesterol, avenasterol, brassicasterol, stigmasterol) and oxyphytosterols (7α- and 7β-hydroxysterol, 5α,6α- and 5β,6β-epoxysterol, 7-ketosterol and triols) were estimated by gas chromatography. In all samples with antioxidants, a lower decrease of phytosterols and a lower increase of total oxyphytosterols were observed in comparison with the control sample (without antioxidant). The antioxidant effect depends on the type of the antioxidant and the heating conditions. The best results were observed in samples with natural antioxidants. BHT was a substance that protected phytosterols as well, but not as effectively as the other antioxidants.